Metallothermal process and continuous drawing, in cold induction furnaces, of metals or alloys

ABSTRACT

A metallothermy method includes continuously withdrawing an ingot of a metal product such as uranium or a uranium alloy as it is formed from an oxide or salt of the metal. The method employs upper and lower communicating crucibles. The upper crucible contains a layer of reducing medium floating on a solvent medium. The metal oxide or metal salt is reduced as it is introduced into the layer reducing medium to form metal particles and slag. The settling metal is melted by further induction heating, collected in the lower crucible and withdrawn. The slag is absorbed by the solvent medium which is regenerated with the use of electrolysis.

TECHNICAL FIELD

This invention relates to the metallothermy and continuous drawing offof metals or alloys, in cold inductive crucibles. The metals or alloysare produced from metallic oxides or salts. The invention particularlyconcerns the preparation of uranium metal from a uranium oxide or salt.It also concerns the preparation of an alloy of uranium or anothermetal.

State of Prior Art

Conventionally, uranium metal is obtained after transformation of UO₂oxide into UF₄, followed by a step to reduce the tetrafluoride intouranium metal. No current industrial process is capable of directlyproducing solid uranium metal ingot from the dioxide. The action of areducing metal such as Ca or Mg on uranium dioxide, despite its verystrong exothermal nature, cannot melt the lime or magnesia, and uraniummetal is obtained divided in an unmeltable solid. The slag may bedissolved in an acid, but the processing cost is prohibitive for largescale production and there is a risk that the divided metal could beattacked. On the other hand, the slag composed of fluorite or sellaiteformed during reduction of UF₄ by these metals is relatively meltable.The uranium assembles in situ in the form of an ingot. Slag generates asolid waste, treated by a wet method before being permanently stored ina controlled tip.

At the present time, constraints necessary for the reduction of wasteinvolve the use of recycling processes, for example using electrolysis.

Patent U.S. Pat. No. 5,290,337 divulges a process for the reduction ofuranium oxide by magnesium capable of recovering uranium metal andregenerating the metallic reduction agent. Uranium oxide (for exampleUO₂) is reduced in the presence of molten salts, for example MgCl₂/MgF₂or MgCl₂/NdCl₃ in order to improve the settlement of uranium metal, asrecommended by C. MORANVILLE and J. DUBUISSON in Le Nouveau Traité deChimie Minérale (The New Treatise of Inorganic Chemistry) publishedunder the management of P. PASCAL, Volume XV, book 1, 1960, Masson andCompany, page 187, and J. H. BUDDERY in Metallurgy and Fuels, 1956, vol.4, pages 24-32. The metal is recovered in a layer of molten ZnMg or CuMgalloy. UO₂ is added into the two-phase medium composed of the moltenalloy (ZnMg) and the mix of molten salts (MgCl₂/NdCl₃), stirred andheated to 750° C. Some of the magnesium reduces UO₂ by forming U andMgO. The MgO then reacts with neodymium chloride to produce magnesiumchloride and neodymium oxide. The uranium metal is incorporated in theZnMg phase. When the reactional medium is no longer stirred, the phasesseparate. The denser metallic phase settles at the bottom of thecrucible. After cooling and solidification, the phases are mechanicallyseparated. The uranium metal is then separated by evaporation of ZnMg.Mg is regenerated by electrolysis of the salt.

Thus according to U.S. Pat. No. 5,290,337, reduction of UO₂ by Mg takesplace in a two-phase mix while being stirred. The principle of twoseparate phases only appears after the end of the process that can onlybe a discontinuous system (“batch process”). Therefore, this process hasthe disadvantage that it cannot directly and continuously produceuranium metal at the output from the crucible. Two additional steps arenecessary to separate the constituents, firstly a mechanical separationof the phases and then a distillation. Furthermore, the bath isnecessarily remelted before electrolysis and regeneration of Mg, and itcannot subsequently be recycled unless an additional step is added toconvert the neodymium oxide into chloride.

Since the 1960s, many studies, publications and patents have been madeon continuous melting and drawing off of ingots. The metal is usuallyadded directly into the drawing off crucible, after being produced inother structures, for example by electrolysis, metallothermy orreduction by a metalloid (carbon, sulfur).

The “Economically Producing Reactive Metals by Aerosol Reduction”article by J. D. LELAND, published in the J.O.M. review, pages 52-55,October 1996, divulges a process for obtaining titanium, hafnium orzirconium semi-continuously from their precursor salts (chlorides). Thereduction process by aerosols consists of a reaction between two jets ofproducts.

The reducing metal (for example Na or Mg) is in the form of an aerosolbetween 400 and 600° C., and the metal chloride to be reduced is invapor form. Due to the exothermal nature of the reaction, the chlorideformed is vaporized whereas the solid metal drops to the bottom of aliquid metal bath. The temperature of the medium is stabilized at aboutthe boiling point of the salt formed, typically 1100-1200° C. The metalis collected and molten starting from the bottom of the bath in aninductive cold crucible. This process has two main disadvantages,firstly the necessity to have an exothermal reaction in order tovaporize the slag and the initial product that has not reacted, and thedifficulty in controlling feed flows. Furthermore, this process wouldnot be applicable in the case of exothermal reactions in which there isa risk of reaching the thermodynamic inversion temperature or if theslag boiling temperature exceeds the thermodynamic inversiontemperature.

DESCRIPTION OF THE INVENTION

The invention overcomes the disadvantages of prior art mentioned aboveby proposing a solution that enables complete and continuous productionas far as the preformed metal, starting from the metal salt or oxide.

Therefore, the purpose of the invention is a metallothermy andcontinuous drawing off process for a metal product, composed of at leastone metal, comprising:

a metal production step in a first cold crucible heated by induction inwhich reduction of the oxide or salt of the said metal is provoked in areducing medium composed of a floating layer of melting material and inwhich the formed metal settles in a solvent medium consisting of a bathcomposed of at least one molten salt that absorbs the slag resultingfrom the reduction reaction, the maximum input flow of the floatinglayer of oxide or salt of the said metal being determined by thethickness of the floating layer and the temperature of its top surfacesuch that the reduction of the oxide or salt of the said metal takesplace entirely in the floating layer;

a step in which the settled metal is collected and melted, that takesplace in a second cold crucible heated by induction located under thefirst cold crucible, in order to enable continuous drawing off of themetallic product.

This determination of the maximum feed flow may depend on test resultsand/or a model to produce nomograms. The floating reducing medium can beused to perfectly control the temperature and avoid reversibility of thereactions.

Induction heating of the first crucible and the second crucible can takeplace at different frequencies.

The reducing medium may include a material chosen among a metal, a mixof metals, a metalloid or a mix of metalloids.

The oxide or the salt of the said metal to be produced may be added intothe first cold crucible above the layer if this oxide or salt is in thesolid state or the liquid state. The oxide or salt of the said metal tobe produced may be added into the first cold crucible under the layer ifthis oxide or salt is in the gaseous state.

If the metallic product to be produced is an alloy of at least twometals, the first cold crucible may be fed by a mix of oxide(s) orsalt(s) of these two metals. As a variant, one of these metals may beadded into the first cold crucible directly in its metallic form, theother of these metals being added in the form of an oxide or salt.

Advantageously, the process also comprises a step in which the reducingmedium is regenerated by electrolysis of the slag present in the solventmedium. The solvent medium is drawn off continuously.

Another purpose of the invention is a device for embodiment of theprocess described above, characterized in that it comprises:

a first crucible with cold walls equipped with induction heating meansand means of adding the oxide or salt of the said metal to be producedinto the reducing medium.

a second crucible with cold walls equipped with induction heating meanslocated under the first crucible, the upper part of the second cruciblecommunicating with the lower part of the first crucible, the lower partof the second crucible being provided with means of continuouslydraining the metallic product.

Preferably, this first crucible has a larger cross sectional area thanthe second crucible. In this case, the junction between the twocrucibles may be formed by a board of refractory material, for example aceramic board.

Preferably, the induction heating means are located on the first andsecond crucibles such that it forms a settlement cone composed of thesolid solvent medium, under the influence of the cold walls of thecrucibles.

Advantageously, the first crucible is equipped with means of drawing offthe solvent medium, operating continuously. It is also equipped withmeans of adding the reducing medium, operating continuously. It is alsoequipped with means of adding the oxide or salt of the said metal to beproduced, operating continuously.

The process according to this invention is particularly applicable toobtaining uranium metal.

If the uranium compound that is reduced is UO₂ oxide or U₃O₈ oxide, thereducing medium may consist of lithium, and the solvent medium maycomprise at least one of the following salts: LiCl, KCl, BaCl₂, LiF,CaF₂ and BaF₂. If the uranium compound that is reduced is UF₄, thereducing medium may be composed of a metal chosen among Ca, Mg, Li, K,Na or a Ca-Mg mix, the solvent medium may comprise at least one of thefollowing salts: MgF₂, MgCl₂, LiCl, KCl, BaCl₂, LiF, KF, CaCl₂, CaF₂,NaF, NaCl and BaF₂. If the uranium compound that is reduced is a doublefluoride of uranium and an alkali or alkali earth element, the reducingmedium may be a metal chosen among Ca, Mg, Li, Na or K, or a mix of atleast two of these reducing metals, and the solvent medium may includeat least one of the following salts: MgF₂, MgCl₂, LiCl, KCl, NaCl, NaF,BaCl₂, LIF, CaF₂, CaCl₂ and BaF₂. If the uranium compound that isreduced is Cs₂Ucl₆, the reducing medium may be a metal chosen among Ca,Mg or Li or a mix of at least two of these reducing metals, the solventmedium may comprise at least one of the following salts: LiCl, KCl,CsCl, BaCl₂, LiF, CaF₂, BaF₂, MgF₂ and MgCl₂.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood, and other advantages andspecific features will become clear after reading the followingdescription, given as a non-restrictive example, accompanied by theattached FIGURE which is a longitudinal sectional view through a devicecomprising two crucibles with cold walls used for the embodiment of theprocess according to this invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

Metallothermy and drawing off of metals or alloys according to theinvention take place in two cold inductive crucibles with differentsizes (height, diameter, geometry). The innovation lies in the fact thatthe two crucibles are superposed and that the two steps of the processare perfectly continuous.

The metal is produced by reduction of an oxide or a salt of this metalin the upper crucible. For example, the reducing agent may be a metal ora mix of metals with a higher reducing capacity than the metal to beproduced, or a metalloid (carbon or sulfur). The metal formed is settledin a solvent medium, for example a molten salt or a liquid metal that isimmiscible with the reduction agent or the metal formed. The metal iscollected and the ingot is drawn off in the lower cold inductivecrucible, the diameter or the cross-section of the crucible beingdetermined as a function of the required form (wire, bars, etc.).

The process according to the invention is applicable particularly forobtaining metal by metallothermic reduction and separation in a moltensalt bath. The salt or oxide is reduced in a layer of melting reducingmetal (pure or a mixture) floating on a molten salt bath. The reductionagent is chosen for its reduction capacity, its density and for itsinability to form an alloy with the metal to be produced. Therefore,this metal passes through the layer of reducing metal and then settlesin the molten salt bath. The molten salt is determined as a function ofseveral criteria including its ability to solubilize the slag, itsdensity, its stability with respect to the various metals present, andits melting and boiling temperatures. Since the slag is solubilized bythe salt, metal is obtained pure at the bottom of the reaction crucibleand is molten and then drawn off in the lower inductive cruciblestarting from the bottom of the liquid bath.

A variant of the process consists of reducing a mix of salts or oxidesof different metals and directly drawing off an alloy of these metals.One of these metals may also advantageously be added in solid form(metallic powder).

Unlike prior art, the use of a buffer area of molten salt (denser thanthe reducing metal and less dense than the metal to be recovered) tophysically separate the reducing and melting areas, makes it possible toconsider the use of reactions for which the thermodynamic inversiontemperature would be reached in a “batch” system due to the exothermalnature or melting of the metal produced. The intermediate salt bathmakes it possible to impose a temperature gradient on the system suchthat the temperature is higher at the bottom of the first crucible.Furthermore, since the reaction takes place in a cold inductivecrucible, the heat generated by the reaction in the layer of reducingmetal can be dissipated. Therefore the exothermal nature of the reactionconsidered can be controlled. It is also important to note that thereaction does not have to be highly exothermal in this process.

The attached FIGURE shows a longitudinal cross-sectional view of thecold crucibles used for embodiment of the process according to theinvention, for example for reduction of an oxide by a reducing metal. Itcomprises a first cylindrically shaped cold crucible 1 on top of asecond cold crucible 3, also cylindrically shaped but with a smallerdiameter. Crucibles 1 and 3 are fitted together through a ceramic board11 on which the crucible 1 is placed. An orifice in board 11 enablesplacement of the second cold crucible 3. The proximity of the coolingsystem for the two cold crucibles enables the formation of the automaticsettlement cone 15 composed of a solid salt. A high frequency inductioncoil 2 is located around the first crucible 1 and a medium frequencycoil 4 is located around the second crucible 3. According to theinvention, the first crucible 1 is filled with molten salts 7 on top ofwhich floats a layer of reducing metal 8. The metal compound to betreated 12 is added in solid form (for example powder 9), or liquid formon the layer of reducing metal 8. Solid particles of the produced metal6 settle in the liquid salts bath 7 and are collected in the secondcrucible 3 and are then molten and drawn off in the form of an ingot 5.The ingot is continuously drained through the orifice 16. The salt bath7 solubilizes the slag (or sub-product) and is evacuated through theorifice 10 formed in the crucible 1. The height of the drawing offorifice must be determined to avoid entraining the required metal. Theelectromagnetic pressure forces exerted on the decanting metal can avoidthis phenomenon. However, a filter 18 may be added on the drawing offpipe to retain the metal to be produced in the first crucible. In orderto prevent the material from solidifying in the drain pipes between thebath and the electrolysis unit (not shown), the pipes are provided withinductive or resistive heating systems 17. After electrolysis of thebath and the slag (not shown), the reducing metal 14 and the salt 13forming the bath, are added back into reaction vessel 1.

If the required metal is uranium, the process as described can be usedto obtain this metal particularly from oxides (UO₂, UO₃, or U₃O₈) orfluoride (UF₄, double fluoride). The reducing metal is Ca, Mg, or Lialone or mixed for oxides, and Na, Li, K, Ca or Mg alone or mixed forfluorides. For UF₄, or a double uranium fluoride, the possibility of aliquid phase feed will be very advantageous compared with prior art. Thereduction reaction will then take place in a single phase liquid-liquidsystem.

Apart from reactivity constraints with the compound to be reduced, thechoice of the reducing metal depends on its density in the liquid phasecompared with the density of the molten salts bath, and the followingconsiderations. The melting temperature of the reducing medium must beas low as possible to enable circulation in the liquid phase between theelectrolysis unit and the reaction vessel, and it must be as differentas possible from the thermodynamic inversion temperature (for example Lireduces UO₂ below 850° C., and U reduces Li₂O at higher temperatures).The boiling temperature of the reducing metal will be sufficiently highto enable a large choice of compatible salts. The important points forsalts are the melting temperature, their lack of reactivity with thevarious metals present within the temperature range considered, andtheir ability to integrate (solubility or mix) a large proportion ofslag. This latter point has a controlling influence on the sizing of theassociated electrolysis unit to the extent that it is industriallypreferable to avoid the use of a system close to the diffusion limit ofthe compound to be electrolyzed. The boiling temperature of the saltbath will necessarily exceed the melting temperature of the reducingmetal and of the metal produced at the design operating pressure.

Allowing for all these technical considerations, the use of Li as areducing metal is attractive for the reduction of uranium dioxide.Various mixes based on LiCl, KCl, BaCl₂, BaF₂, LiF or CaF₂ may besuitable for the salt bath. The selected mixes, binary or at mostternary, have melting temperatures below 500° C., which can give anLi—salts interface temperature of the order of 600° C.

The process according to this invention may be used to produce metalsother than uranium, either alone or in a mix starting from their oxidesor salts. These metals are particularly Sc, Y, La, Ga, Tl, La, the rareearths (Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), Be, Ti,Zr, Hf, Si, Ge, V, Nb, Ta, Cr, Fe, Pu, etc. This list is not exhaustive,and all elements in the periodic table can be used within the frameworkof the invention.

Similarly, reducing agents other than alkalis or alkali earths can beused. The objective is to use metals that have a higher reducingcapacity than the metal to be reduced, or metalloids like carbon orsulfur.

The compound to be treated can be added in solid or liquid form abovethe metallic layer, or as a gas under this layer.

When materials are produced using metalloids, the mix of the moltensalts may act as a solvent for the reaction. The metalloid is dispersedin the bath that then behaves like a fluidized bed.

The process is also applicable for the production and concentration bymelting, of materials other than pure metals or bodies from their oxidesor salts. For example, the recuperation of boron carbide spongesstarting from boron and carbon oxides.

What is claimed is:
 1. A metallothermy method for continuously formingan ingot of a metal from a metal salt or a metal oxide of said metalcomprising the steps of: a) providing a first cold crucible heated byinduction and a second cold crucible heated by induction, said secondcrucible being in fluid communication with and located under said firstcold crucible to continuously form said ingot, b) forming a layer of amelting material composed of a reducing medium floating on a solventmedium composed of at least one molten salt in said first cold crucible,c) introducing said metal salt or metal oxide into said layer, d)reducing said metal salt or metal oxide entirely in said layer andforming particles of slag and of said metal, e) settling said metalparticles downwardly through said solvent medium in said first coldcrucible to pass said metal particles into said second cold crucible andsolubilizing said slag in said solvent medium, f) collecting and meltingsaid metal particles in said second cold crucible to form said ingot, g)continuously drawing off said ingot from said second cold crucible. 2.The method of claim 1, wherein said induction heating of said first andsecond crucibles is done at different frequencies.
 3. The method ofclaim 1, wherein said reducing medium includes a material selected fromthe group consisting of a metal, a mix of metals, a metalloid or a mixof metalloids.
 4. The method according to claim 1, wherein said metalsalt or metal oxide is in a solid state or a liquid state and added tosaid first cold crucible above said layer.
 5. The method according toclaim 1, wherein said metal salt or metal oxide is in a gaseous stateand added to said first cold crucible under said layer.
 6. The method ofclaim 1, wherein said metal of said ingot is an alloy of at least twoalloying metals, and said metal salt or metal oxide is a mix of oxide(s)or salt(s) of the two alloying metals.
 7. The method of claim 1, whereinsaid metal of said ingot is an alloy of at least two alloying metals,one of the alloying metals being added into said first cold crucibledirectly in its metallic form, the other of the alloying metals beingadded in the form of an oxide or a salt.
 8. The method of claim 1,further including regenerating said reducing medium by electrolysis ofsaid solvent medium.
 9. The method of claim 8, wherein said step ofregenerating said reducing medium by electrolysis includes continuouslywithdrawing said solvent medium to be regenerated by electrolysis. 10.The method of claim 1, wherein said metal of said ingot is uranium metalor a uranium alloy.
 11. The method of claim 10, wherein said metal oxideis UO₂ oxide or U₃O₈ oxide, the reducing medium consists of lithium, andthe solvent medium comprises at least one salt selected from the groupconsisting of LiCl, KCl, BaCl₂, LiF, CaF₂ and BaF₂.
 12. The method ofclaim 10, wherein said metal salt is UF₄, the reducing medium iscomposed of a metal selected from the group consisting of Ca, Mg, Li, K,Na or a Ca-Mg mix, the solvent medium comprises at least one saltselected from the group consisting of MgF₂, MgCl₂, LiCi, KCl, BaCl₂,LiF, KF, CaF₂, CaCl₂, NaF, NaCl and BaF₂.
 13. The method of claim 10,wherein said metal salt is a double fluoride of uranium and an alkali oralkali earth element, the reducing medium is a metal selected from agroup consisting of Ca, Mg, Li, Na or K, or a mix of at least two ofthese reducing metals, and the solvent medium comprises of at least onesalt selected from the group consisting of MgF₂, MgCl₂, LiCl, KCl, NaCl,NaF, BaCl₂, LiF, KF, CaF₂, CaCl₂ and BaF₂.
 14. The method of claim 10,wherein said metal salt is CS₂UCl₆ the reducing medium is a metalselected from the group consisting of Ca, Mg or Li or a mix of at leasttwo of the e reducing metals, the solvent medium comprises at least onesalt selected from the group consisting of LiCl, KCl, CsCl, BaCl₂, LiF,CaF₂, BaF₂, MgF₂ and MgCl₂.
 15. The method of claim 11, wherein steps(c) and (d) include determining a maximum input flow of said metal saltor metal oxide based on the thickness of said layer and the temperatureat a top surface of said layer to assure reducing said metal salt ormetal oxide entirely in said layer and forming particles of said metaland slag.